1. Field of the Invention
The present invention is directed to a computer tomography apparatus of the type having a circular anode and a circular radiation detector surrounding a measuring opening in which a patient to be examined is disposed so that the patient is irradiated with x-radiation from different angular positions.
2. Description of the Prior Art
Computer tomography devices are known in the art wherein a patient is disposed inside of a circular anode and a circular radiation detector. An electron beam is generated and is deflected by a beam deflection system in a circular orbit, on the circular anode, around the patient, so that the patient is irradiated by a fan-shaped x-ray beam from different angular directions. The radiation attenuated by the patient is recorded by the radiation detector, consisting of an array of individual detector elements, and the measured values from the detector elements are supplied to a data acquisition system, from which the measured values are supplied to a computer which constructs an image of a slice of the examination subject from those values. The image is then visually displayed.
Other types of computer tomography devices are known wherein the x-ray source and the radiation detector are mechanically moved around the examination subject, so as to expose the patient to radiation from the different angular positions. In tomography devices of this type, if a tomogram of a beating heart is needed, measured values of the attenuated radiation during a plurality of heart cycles must be selected in order to achieve images of the beating heart which are low in artifacts. The measured values are always produced at the same heart phase, i.e., at the same time within the heart beat cycle. Fluctuations in the line voltage which supplies high-voltage generator for the x-ray source will result in variations of the intensity and mean energy of the x-radiation and in migrations of the x-ray beam position as it exits the x-ray source. Beam position monitors in this type of tomography apparatus can be mechanically mounted at the beam exit port of the x-ray source, and thus co-moved with the x-ray source around the patient. The detector then supplies a signal to the computer which is used to correct for voltage variations, so that the change in the intensity and mean energy of the x-ray beam caused by the voltage fluctuations can be taken into account in the computerized construction of the image, and image artifacts, which would otherwise be caused by this voltage fluctuation, can be avoided.
A computer tomography apparatus of the type first described above, i.e., having a circular anode on which an electron beam orbits around a patient, is described in U.S. Pat. No. 4,352,021. In this type of computer tomography apparatus, a very fast movement of the focus around the circular anode is possible, so that the registration of heart phases is possible during a single heart cycle. The focus of the electron beam is conducted around the circular anode using an electrical and/or magnetic beam deflection system. Therefore mechanical parts for moving the focus are not present in this type of tomography apparatus, and the aforementioned mechanical radiation monitors cannot be used, and thus image artifacts caused by fluctuations in the line voltage are present.